The present invention relates generally to methods and devices for balancing the circuits of a three-phase system, and more particularly to such a method and a device wherein the three-phase system is produced by means of an invertor from a d.c. voltage source such that the output terminals of this invertor are equipped with a filter with a neutral grounding transformer.
A device for the production of a symmetrical three-phase initial power system with a mid-point conductor is known from European patent 0 208 088. This device contains an array of invertors and a voltage transformer. The transformer is a potential-free three-phase transformer with secondary neutral point circuitry that is attached to the invertor arrangement. In addition, a filter is wired in series with the transformer device. An ideal level indicator prescribes three symmetrical angles as setpoints. A regulating device, to which a set of controls is attached, is assigned to each setpoint value and each output terminal. A pulse-width modulated, triple-phasic bridge voltage changer is intended to serve as an invertor arrangement. A decoupling network that depicts the structure of the transformer device forms, from transient values that correspond to a voltage system at the input terminals of the voltage transformation device. The regulatory arrangement and the decoupling network are wired together in such a way that they provide control voltage for the pairs of branches that each work on one output terminal of the invertor from the setpoint readings and measurements for the phase-to-neutral voltages of the initial current system such that by means of the control voltage, the deviation of the transformed measurements from the setpoint measurements can be regulated. By means of this device, fleeting asymmetries that occur at the clamps of the initial voltage system can be adjusted quickly because each pair of the invertor arrangement is powered by a set of controls in the nature of an asymmetrical power system with a controlling voltage that is quickly changed. The decoupling network makes it possible to convert those measurable variables that are secondarily connected to the load-bearing star point into substitute measurable variables of a zero-conductor free system. As a result, substitute actual values stand ready at the input of the current transformer device for regulation. The control currents that are formed are asymmetrical.
The present invention is directed to the problem of developing a method and a device for producing symmetry in a three-phase system, without resorting to a conversion of secondary measurable variables connected to a star point capable of bearing a load.